DOS Plus

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DOS Plus (also known as DOS+) is an operating system written by Digital Research, first released in 1985. It can be seen as an intermediate step between CP/M-86 and DR-DOS.

It is able to run programs written for either CP/M-86 or MS-DOS 2.11, and can read and write the floppy formats used by both these systems. Up to four CP/M-86 programs can be multitasked, but only one DOS program can be run at a time.

1 User interface
2 Commands
3 Internal structure
4 Computers that used DOS Plus
5 References

User interface

DOS Plus attempts to present the same command line interface as MS-DOS. Like MS-DOS, it has a command line interpreter called COMMAND.COM. There is an AUTOEXEC.BAT file, but no CONFIG.SYS. The major difference the user will notice is that the bottom line of the screen contains status information similar to:

DDT86    ALARM                  UK8 PRN=LPT1      Num    10:17:30

The left-hand side of the status bar shows running processes. The leftmost one will be visible on the screen; the others (if any) are running in the background. The right-hand side shows the keyboard layout in use (UK8 in the above example), the printer port assignment, the keyboard caps lock and num lock status, and the current time. If a DOS program is running, the status line is not shown. DOS programs cannot be run in the background.

The keyboard layout in use can be changed by pressing Ctrl, Alt and one of the function keys F1-F5.

Commands

DOS Plus contains a number of extra commands to support its multitasking features:

ADDMEM: Sets the amount of extra memory to allocate to EXE programs.
ALARM: A message alarm clock.
BACKG: Allows background processes to be listed and stopped.
COMSIZE: Sets the amount of memory to allocate to COM programs.
PRINT: Print spooler.
SLICE: Sets the amount of processor time to give to the foreground program.
USER: Sets the user number to use when accessing CP/M media.

It also contains subsets of the standard DOS commands and CP/M commands - for example, it has both a built-in COPY command, and a PIP utility, both of which copy files.

The CD command can assign one of the three drives N:, O: or P: to a directory on a different drive, in a similar manner to the MS-DOS command SUBST. For example,

CD N:=C:\DATA\ACCOUNTS

will cause the directory C:\DATA\ACCOUNTS to appear as drive N:. This unique 'floating drive' feature is also present in DR DOS 3.31 - 6.0 (initial 1991 release only). Interestingly, it is implemented in the BDOS itself, not in the shell.

Internal structure

DOS Plus boots from a single file called DOSPLUS.SYS (rather than the IO.SYS / MSDOS.SYS combination of MS-DOS). This file is in the CP/M-86 CMD format, and is structured internally as a number of modules:

The BDOS kernel, which handles multitasking and implements the CP/M API.
The XIOS, which is the machine-dependent component that performs low-level disc and character I/O.
The DOS emulator, which converts each MS-DOS function call into one or more CP/M calls. In the case of a request to change the current directory, for example, this would translate to a series of 'open directory' calls, one for each directory in the path.
A stub which loads COMMAND.COM when required. This stub contains the string "OS=CPCDOS", suggesting that it is based on the Concurrent DOS codebase.